1. Field of the Invention
The present invention relates to a method and apparatus for the measurement of the refractive index of a gas. More particularly, the invention relates to optical apparatus which is useful for high accuracy displacement metrology using interferometry in ambient air.
2. The Prior Art
An interferometer is the basic instrument for most of the high-accuracy displacement measurements in the machine tool and semiconductor fabrication industries. One type of interferometer representative of the current state of the art is described in Bagley et al., U.S. Pat. No. 3,458,259 issued July 26, 1969. The absolute accuracy of interferometric displacement metrology is limited by two dominant factors: (1) the uncertainty in the vacuum wavelength of the light source, and (2) the uncertainty in the refractive index of the ambient air, see W. Tyler Estler, "High-Accuracy Displacement Interferometry in Air:," Applied Optics, vol. 24, pp. 808-815 (Mar. 15, 1985) and Farrand et al., U.S. Pat. No. 4,215,938 issued Aug. 5, 1980.
As noted in the aforementioned references, interferometric displacement measurements in air are subject to environmental uncertainties, particularly to changes in air pressure, temperature, humidity, and molecular composition. Such factors alter the wavelength of the light used to measure the displacement. Under normal conditions the refractive index of air is approximately 1.0003 with a variation of .+-.10.sup.-4. In many applications the refractive index of air must be known with an error of less than 10.sup.-7 to 10.sup.-8.
One prior-art technique for correcting the environmental uncertainties is based on using individual sensors to measure the barometric pressure, temperature, and humidity, and, then, using these measurements to correct the measured displacement. The commercially available Automatic Compensator, Model 5510 Opt 010, from Hewlett-Packard uses this technique. This technique has been only partly satifactory due to the errors in the sensors and due to the errors arising from variations in the composition of the air, e.g., the percentage CO.sub.2 content and presence of industrial gases, i.e. Freon and solvents are ignored in this technique.
A second prior-art technique is based on the aforementioned Farrand et al., U.S. Pat. No. 4,215,938 issued Aug. 5, 1980. This technique incorporates a rigid enclosure, the length of which must be accurately known, independent of environmental conditions and constant in time. The change in optical path length of this enclosure is measured as remotely controlled valves allow the enclosure to be evacuated and refilled with ambient air. The wavelength of the air in the enclosure is proportional to the measured change in optical path length. This technique has also been only partly satisfactory due to the fact that the characteristics of the air in the enclosure do not adequately represent those of the air in the measurement path, thusly systematic errors are introduced. It has been found that even with a perforated enclosure, serious systematic differences exist between the characteristics of the air inside of and external to the enclosure. In addition, the need for valves and a vacuum pump makes this technique awkward to implement for many applications.
Another prior-art technique incorporates a fixed length optical reference path which contains the ambient air. The technique measures the difference in optical length of the fixed length due to the variations in the refractive index of the ambient air. This technique is only partly satisfactory due to the fact that since it is differential it depends critically on the precise knowledge of the initial conditions.
Consequently, while prior-art techniques for measuring the refractive index of a gas are useful for some applications, none known to the applicant provide the technical performance in a commercially viable form for applications requiring the high accuracy interferometric measurement of displacement in air. The disadvantages of the prior-art apparatus are overcome by the present invention.